Dual clutch transmission

ABSTRACT

A dual clutch transmission, especially of a motor vehicle, includes a hydraulic circuit with a pump for delivering a hydraulic flow, a cooler for cooling the hydraulic flow, and a volume control valve for adjusting the hydraulic flow for a cooling system associated with two clutches of the dual clutch transmission. The volume control valve is constructed as a switching valve with at least two switching position ranges, supplying the hydraulic flow to a first cooling system associated with a first clutch in a first switching position range with a constant flow cross-section, and supplying the hydraulic flow to a second cooling system associated with a second clutch in a second switching position range with a constant flow cross-section, wherein the entire switching position range of the volume control valve is substantially formed by the first and second switching position ranges and only a narrow transition range therebetween.

The invention relates to a dual clutch transmission, in particular of a motor vehicle, in particular with a hydraulic circuit for cooling the dual clutch transmission, wherein the hydraulic circuit includes at least one pump for conveying a hydraulic medium flow, at least one cooler for cooling the hydraulic medium flow, and a volume control valve for adjusting the hydraulic medium flow for at least one cooling system associated with clutches of the dual clutch transmission.

Dual clutch transmissions are preferably used in passenger cars. A dual clutch transmission generally includes two coaxially disposed transmission input shafts, which are each associated with a sub-transmission. A respective clutch is associated with each of the transmission input shafts, via which the transmission input shaft of the respective sub-transmission can be frictionally coupled to the output of an engine, preferably an internal combustion engine of a motor vehicle. A first of the two sub-transmissions typically includes the odd gears, whereas a second of the two sub-transmissions includes the even gears and the reverse gear.

Typically, one of the sub-transmissions is active while driving, which means that the transmission input shaft associated with this sub-transmission is coupled to the engine via its associated clutch. A gear providing a current gear ratio is engaged in the active sub-transmission. A controller determines whether the next higher or next lower gear is to be engaged depending on the driving situation. This gear which is probably used next is selected in the second, inactive sub-transmission. When changing gears, the clutch of the inactive sub-transmission is engaged, while the clutch of the active portion is disengaged. Preferably, opening of the clutch of the active sub-transmission and closing the clutch of the inactive sub-transmission overlap so that the flow of force from the engine to the drive shaft of the motor vehicle is not at all or only briefly interrupted. As a result of the gear change, the previously active sub-transmission becomes inactive, while the previously inactive sub-transmission becomes the active sub-transmission. Thereafter, the gear expected to be used next can be engaged in the now inactive sub-transmission.

The gears are engaged and disengaged via elements, preferably via the shift rails that are actuated by hydraulic cylinders, i.e. the aforementioned switching cylinders, which have already been mentioned above. The hydraulic cylinders are preferably formed as double-acting hydraulic cylinders, in particular synchronous cylinders or differential cylinders, so that preferably two gears may be associated with each switching cylinder. Alternatively, single-acting hydraulic cylinders may also be provided. The hydraulic cylinders operating the elements, in particular the shift rails, are also referred to as gear selector cylinders. A gear selector cylinder designed as a synchronous cylinder to which in particular two gears are assigned, has preferably three switching positions, wherein in a first switching position a first defined gear is engaged, in a second switching position another defined gear is engaged, and in a third switching position none of the two above-mentioned gears is engaged.

The clutches associated with the two sub-transmissions are also hydraulically actuated, i.e. closed or opened. Preferably, the clutches each close when hydraulic pressure is applied, whereas they open when no hydraulic pressure is applied, i.e. when pressure is relieved in a hydraulic cylinder associated with the respective clutch which is also referred to as clutch cylinder, as mentioned above.

In addition, the operation of a dual clutch transmission is known per se and will thus not be discussed here in detail.

The structure and the operation described in the preceding paragraphs preferably apply also to or are related to the subject matter of the invention.

As already indicated, dual clutch transmissions are controlled or regulated as well as cooled by a hydraulic circuit. This hydraulic circuit, or subassemblies thereof, and methods associated therewith are the object of the invention.

Cooling of the clutches is typically attained with a fixed displacement pump driven by an internal combustion engine. At least one control or control valve is used for adequately cooling the clutch. The accuracy of the flow rate used for cooling the clutch strongly depends on the employed control or control valve. Typically, the hydraulic medium conveyed by the pump is cooled by a cooler and then supplied to a cooling system associated with the clutches, so that only a single volume flow rate of the cooling medium is provided to both clutches. The common cooling of the clutches thereby worsens the control performance of couplings.

Furthermore, EP 1637756 A1 discloses a system where a volume control valve is connected upstream of the clutches, which supplies in a first switching position a lubricant to one clutch and a coolant to the other clutch, and supplies in another switching position a lubricant to one clutch and a coolant to the other clutch. Here, supplying coolant to the clutches depends here always on the lubrication of couplings, so that neither the clutch cooling nor the lubrication can be optimized.

In addition, conventional volume control valves must be calibrated to allow a control command to cause a corresponding state of the volume control valve or a desired distribution/release of one or more hydraulic medium flows. The switching position especially of solenoid valves is determined by a current command, wherein the position and hence the quantity of hydraulic medium is adjusted depending on the magnitude of the current command. However, the calibration of such volume control valves is complicated and correspondingly expensive.

It is thus the object of the invention to provide a dual clutch transmission that provides in a simple and economical way a reliable operation of the volume control valve and thus the supply of the cooling system with chilled hydraulic medium.

The object of the invention is attained in that the volume control valve is constructed as a switching valve having at least two switching position ranges, wherein the hydraulic medium flow is conveyed in a first switching position range with a constant flow cross-section to a cooling system associated with a first of the clutches and in a second switching position range with a constant flow cross-section to a second cooling system associated with a second of the clutches, wherein the total switching range of the switching valve or volume control valve is substantially formed by the switching position ranges and only a narrow transition region between neighboring shift position ranges is provided. The switching position ranges with a constant flow cross-section refer to a respective actuation travel of the switching valve, within which the control valve can be actuated/adjusted without causing the volume flow or a set flow cross-section to change. In other words, the switching valve can be switched into the respective switching position range without changing the switching state with respect to the set fluid connection. The total switching position range represents here the range over which the control valve is overall adjustable. The overall switching position range is divided into the switching position ranges and the respective transition region located between adjacent switching position ranges. According to the invention, the transition region is rather narrow, so that the switching position ranges form substantially the entire switching range, so that the available switching position ranges within which the flow of the hydraulic medium is not affected by actuation of the switching valve are as large as possible. This ensures that the volume control valve of the dual clutch transmission need not be calibrated. The wide switching position ranges ensure that the desired switching position of the switching valve can be attained even when setting a control signal having tolerances and when the switching valve has production-related tolerances. The shifting position ranges are then guaranteed to be greater than the expected tolerances. Because the switching valve is unable to individually adjust the respective flow volume and instead is only intended to open and close a connection, so that there is ultimately a digital operation, a safe operation of the dual clutch transmission is thus guaranteed in a particularly simple and cost-effective manner even without prior calibration of the flow control valve. Preferably, the transition region is designed to be so narrow that an almost constant flow cross-section is attained over the entire switching position range of the switching valve. This prevents backpressure spikes in a switchover.

Adjustment or actuation of the switching valve always refers to actuating or displacing a movable valve element of the switching valve, which cooperates for example with one or more flow-through openings in a stationary housing to open one or more flow cross-sections. Preferably, in the switching valve is a gate valve or rotary vane valve.

According to an advantageous embodiment of the invention, the switching valve is designed as a 4/3-way switching valve or as a 3/2-way switching valve. The 3/2-way proportional valve has three ports and two switching position ranges with a narrow transition, wherein a first port is connected to the pressure side of the pump, a second port is connected to the first cooling system, and a third port is connected to the second cooling system. The 4/3-way switching valve has at least one additional port, and an additional third switching position range, wherein the additional port is connected to a return conduit leading to a tank providing the hydraulic medium.

Preferably, the switching valve embodied as a 4/3-way switching valve conveys the hydraulic medium flow in a third shifting position range to a tank providing the hydraulic medium. When the volume control valve is disposed downstream of the cooler, the hydraulic medium can be cooled and returned to the tank by switching the switching valve into its third shifting position range, thereby generating a small cooling circuit for cooling the hydraulic medium residing in the tank.

Preferably, the volume control valve and the switching valve, respectively, can be controlled by an electric motor and/or electromagnetically. For this purpose, an electric motor and/or an electromagnetic actuator is advantageously associated with the volume control valve, which can then be brought quickly and accurately to the desired shifting position range. Due to the advantageous embodiment of the switching valve with shifting position ranges having a constant flow cross section, wide flow ranges are available within which the switching valve can be brought into the desired switching position. The wide distribution of the current ranges eliminates the need for calibration, since the flow ranges are for switching the positions or position ranges are greater than the expected tolerances.

Preferably, the pump is operatively connected to an in particular variable-speed electric motor. The design of the volume control valve as a switching valve initially eliminates the possibility to affect the quantity of the hydraulic medium flow. By providing the electric motor, the capacity of the pump and hence the flow volume and the resulting quantity of the conveyed hydraulic medium can be adjusted so that preferably the electric motor is controlled for influencing the quantity of the hydraulic medium flow, wherein in particular the speed of the electric motor is controlled or regulated accordingly.

Preferably, a manually operable separation element is interposed between the pump and the electric motor. Conveniently, the drive shaft of the pump is or can be operatively connected to an output shaft of the drive or of the electric motor by way of the separating element. The separating element is preferably an actuatable clutch or an overrunning clutch. The pump can then be switched off by actuating the clutch or by changing the direction of rotation, so as to interrupt transport of the hydraulic medium.

According to an advantageous embodiment of the invention, the second switching position range may be located between the first switching position range and the third switching position range. The switching position ranges assigned to the cooling systems are then adjacent to each other, so that the cooling systems can be supplied quasi simultaneously with hydraulic medium by pulsed control of the switching valve.

According to an alternative preferred embodiment of the invention, the third switching position range is located between the first switching position range and the second switching position range. This prevents hydraulic medium from flowing to the other cooling system during pulsed actuation of the switching valve for setting a desired hydraulic medium flow for only one of the clutches or cooling systems. Instead, during pulsed control, the hydraulic medium flow that is not conveyed to the respective cooling system is transported into the tank.

Furthermore, the quantity of the hydraulic medium conveyed to the first and/or second cooling system is preferably determined by a pulsed control of the switching valve and/or by adjusting the rotation speed of the electric motor.

The hydraulic circuit according to the invention will now be illustrated with reference to FIG. 1.

FIG. 1 shows a hydraulic circuit 1 which is used to actuate, in particular to couple and engage and disengaging gears of a dual clutch transmission and to cool the transmission. The hydraulic circuit 1 includes a tank 3, serving in particular as a reservoir or sump for a hydraulic medium used for operation and cooling, in which the hydraulic medium is preferably stored without pressure. An electric motor 5 driving a first pump 7 and a second pump 9 is provided. Preferably, the speed and rotation direction of the electric motor 5 can be controlled, preferably regulated. The first pump 7 is fixedly connected to the electric motor 5, i.e. without a separation element. In other words, the pump 7 is always driven when the electric motor 5 is running and the hydraulic medium is preferably conveyed in the same direction in the both rotation directions. The pump 9 is preferably connected to the electric motor 5 by way of a separating element 11. Accordingly, the pump 9 can be decoupled from the electric motor 5, so that the pump 9 is not running when the electric motor 5 is running. The separation element 11 is preferably formed as a clutch or an overrunning clutch, wherein in the second situation the direction of rotation of the electric motor 5 determines whether hydraulic medium is conveyed by the pump 9 or not.

The first pump 7 and the second pump 9 are each connected via a corresponding conduit 13, 15 to a junction 17 into which an additional conduit 19 opens. This additional conduit 19 connects the tank 3 to the junction 17 through a suction filter 21. Overall, inlets of the pump 7, 9 are thus connected to the tank 3 via the conduits 13, 15, the junction 17 and the conduit 19 having the suction filter 21.

The outlet of the first pump 7 is connected to a conduit 23 which leads to a junction 25. The junction 25 is connected to the tank 3 via a pressure relief valve 27. The pressure relief valve 27 can open under overpressure in the direction of the tank 3. Furthermore, a conduit 29, which leads via a pressure filter 31 to a port 33 of a switching valve 35, originates from the junction 25.

The pressure filter 31 may be bypassed by a bypass 37, wherein a differential pressure valve 39 is arranged in the bypass 37, which allows bypassing the filter 31 in the direction of the port 33 under overpressure. The differential pressure valve 39 opens starting at a preset differential pressure across the pressure filter 31.

The switching valve 35 is embodied as a 5/2-way valve, which has four additional ports 41, 43, 45, 47 in addition to the port 33. In a first switching state of the switching valve 35 shown in FIG. 1, the port 33 is connected to the port 41, whereas the other ports 43, 45 and 47 are connected blind, i.e. they are closed. The port 41 opens into a conduit 49 in which a check valve is disposed 51. The conduit 49 leads to a pressure accumulator 53, wherein a pressure sensing device 55 is hydraulically connected to the conduit 49 upstream of the pressure accumulator 53.

In a second switching state of the switching valve 35 illustrated in FIG. 1, the port 33 is connected to the port 43 which opens into a conduit 57 that leads to a hydraulic sub-circuit 59 which is used, in particular, to cool the clutches of the dual clutch transmission. In this second switching state, the port 41 is connected blind, and the port 45 is connected to the port 47. In this case, a conduit 61 opens into the port 45 which is subjected to the pressure of the hydraulic medium in the pressure accumulator 53. The port 47 opens into a conduit 63 which is hydraulically connected to a first valve face 65 of the switching valve 35. A second valve face 67 of the switching valve 35 is permanently subjected to the pressure of the pressure accumulator 53 via a conduit 69.

A conduit 73 branches off from the conduit 49 at a junction 71, from which the conduit 61 branches off at a junction 75, and the conduit 69 branches off at a junction 77. The junction 71 is connected to the check valve 51 on the side facing away from the switching valve 35.

The conduit 73 opens into a junction 79, from which the conduits 81, 83 and 85 originate.

The conduit 81 supplies a first sub-transmission in a sub-transmission circuit 87. The first sub-transmission has a clutch K1. The conduit 81 opens into a port 89 of a switching valve 91 which is constructed as a 3/2-way valve and serves as a safety valve for the clutch K1. In a first illustrated switching state of the switching valve 91, the port 89 is hydraulically connected to a port 93, while a port 95 of the switching valve 91 is switched blind. In a second switching state of the switching valve 91, which can be inferred from FIG. 1, the port 93 is connected to the port 95 and via the latter to the tank 3, while the port 89 is connected blind. As will become apparent below, the clutch K1 is switched unpressurized in this second switching state.

The port 93 is connected to a conduit 97 and via the latter to a port 99 of a pressure control valve 101. The pressure control valve 101 is designed as 3/2-way proportional valve having a port 103 which is connected via a conduit 105 to the clutch K1. The pressure control valve 101 additionally has a port 107 that is connected to the tank 3. In a first extreme state of the pressure control valve 101, the port 99 is connected to the port 103, while the port 107 is connected blind. The full pressure prevailing in the conduit 97 of the hydraulic medium is then applied to the clutch K1. In a second extreme state, the port 103 is connected to the port 107 so that the clutch K1 is unpressurized. The pressure control valve 101 regulates the pressure prevailing in the clutch K1 in a conventional manner through proportional variation between these extreme states. A conduit 109 leads from the clutch K1 via a check valve 111 back to the conduit 97. If the pressure in the clutch K1 rises due to the pressure in the conduit 97, the check valve 111 opens, thereby opening a hydraulic connection between the clutch K1 to the conduit 97 via the conduit 109. A conduit 115 branches off from the conduit 109 at a junction 113 which returns the pressure in the clutch K1 as a control variable to the pressure control valve 101.

The conduit 105 includes a junction 117 for hydraulically operatively connecting a pressure detecting device 119. The pressure prevailing in the clutch K1 is thereby detected by the pressure detecting device 119.

The switching valve 91 is controlled by a pilot valve 121 which is actuated by an electric actuator 123. It is formed as a 3/2-way valve and includes the ports 125, 127 and 129. The port 125 is connected via a conduit 131 to a junction 133 provided in the conduit 81. The port 127 is connected by a conduit 135 to a valve face 137 of the switching valve 91. In a first illustrated switching state of the pilot valve 121, the port 125 is switched blind while the port 127 is connected to the port 129 and via the latter to the tank 3, whereby the valve face 137 of the switching valve 91 is switched unpressurized via conduit 135. Preferably, the pilot valve 121 assumes this switching state when no electric control signal is applied to the actuator 123. In a second assumable switching state of the pilot valve 121, the port 125 is connected to the port 127 while the port 129 is connected blind. In this case, the pressure in the conduit 81 operates on the valve face 137 of the switching valve 91 via the junction 133, the conduit 131 and the conduit 135, causing the switching valve 91 to switch against a biasing force into its second switching state where the port 93 is hydraulically connected to the port 95, so that the clutch K1 is unpressurized. Thus, the switching valve 91 can preferably be operated by way of electrical control of the pilot valve 121, so that the clutch K1 is unpressurized and hence open.

The conduit 83 originating at the junction 79 is used to supply a clutch K2 of a hydraulic sub-circuit 139 of a second sub-transmission. Control of the clutch K2 also includes a switching valve 91′, a pilot valve 121′, and a pressure control valve 101′. The operation is identical to the operation already described in conjunction with the first clutch K1. Reference is therefore made to the corresponding description of the sub-transmission circuit 87. The hydraulic actuation of the clutch K2 corresponds to that of the clutch K1.

The conduit 85 originating at the junction 79 is connected to a pressure control valve 141, via which the pressure of the hydraulic medium in a conduit 143 can be controlled. The operation of the pressure control valve 141 preferably corresponds to the operation of the pressure control valves 101, 101′, thus making repeating the description unnecessary. The conduit 143 is connected to a junction 145, from which a conduit 147 and a conduit 149 originate. A junction 151 at which a conduit 153 originates is provided in the conduit 149, via which the pressure prevailing in the conduit 149 and thus also in the conduit 143 is returned to the pressure control valve 141 as a control variable, It is apparent that the junction 151 may also be provided in the conduits 151 or 147.

The conduit 147 is used to supply gear actuator cylinders 155 and 157 in the sub-transmission circuit 87, which are constructed as two double-acting cylinders, i.e. synchronizing cylinders.

A volume control valve 159 is provided for hydraulic actuation of the gear actuating cylinder 155, which is designed as a 4/3-way proportional valve. It has four ports 161, 163, 165 and 167. The first port 161 is connected to the conduit 147, the second port 163 is connected to a first chamber 169 of the gear actuating cylinder 155, the third port 165 is connected to a second chamber 171 of the gear actuating cylinder 155 and the fourth port 167 is connected to the tank 3. In a first extreme state of the volume control valve 159, the first port 161 is connected to the second port 163, while the third port 165 is connected to the fourth port 167. The hydraulic medium can then flow from the conduit 147 into the first chamber 169 of the gear actuating cylinder 155, while the second chamber 171 is connected via the ports 165, 167 to the tank 3 without an applied pressure. A piston 173 of the gear actuating cylinder 155 is then moved in a first direction, for example for disengaging a specific gear of the dual clutch transmission or for engaging another specific gear.

In a second extreme state of the volume control valve 159, both the port 163 and the port 165 are connected to the port 167, while the port 161 is connected blind, In this way, both chambers 169, 171 of the gear actuating cylinder 155 are connected to the tank 3 and unpressurized. The piston 173 of the gear actuating cylinder 155 then remains in its current position due to the absence of forces.

In a third extreme state of the volume control valve 159, the port 161 is connected to the port 165 and the port 163 is connected to the port 167. Hydraulic medium then flows from the conduit 147 into the second chamber 171 of the gear actuating cylinder 155, and the first cylinder chamber 169 is unpressurized in relation to the tank 3 via port 163 and the port 167. The hydraulic medium then exerts a force on the piston 173 of the gear actuating cylinder 155 such that the piston 173 is displaced in a second direction opposite to the first direction. In this way, the aforementioned specified other gear can be disengaged, or the aforementioned specified gear can be engaged.

As described above, the volume control valve 159 is designed as a proportional valve. The hydraulic medium flow coming from the conduit 147 is divided into the chambers 169, 171 by varying the valve states between the three extreme states, so that a defined speed for engaging or disengaging a gear can be specified by controlling/regulating the volume flow.

A conduit 177 which opens into a volume control valve 179 used to control the gear actuating cylinder 157 branches off from the conduit 147 at a junction 175. The operation of the hydraulic control of the gear actuating cylinder 157 is then identical to that described in conjunction with the gear actuating cylinder 155, making a renewed description unnecessary.

The conduit 149 is used to supply gear actuating cylinders 155′ and 157′ of the second sub-transmission in the sub-transmission circuit 139, which are also controlled by volume control valves 159′ and 179′. The sub-transmission circuits 87 and 139 for controlling the gear actuating cylinders 155, 155′ and 157, 157′, respectively, are constructed identically, so reference is made to the preceding description.

The outlet of the pump 9 is connected to a conduit 181 which leads to the hydraulic sub-circuit 59 which is preferably used in particular for cooling the clutches K1, K2. The conduit 181 runs via a cooler 183 to a volume control valve 185. A junction 187 is arranged in the conduit 181 downstream of the outlet of the pump 9 and upstream of the cooler 183, from which a conduit 189 branches off, with the conduit 189 leading to the tank 3 via a pressure relief valve 191 opening toward the tank 3. A junction 193 into which the conduit 57 opens is arranged downstream of the junction 187 and upstream of the cooler 193, with the conduit 57 coming from the switching valve 35 and being connected to its port 43. The hydraulic sub-circuit 59 can be supplied via the conduit 57 with hydraulic medium conveyed by the pump 7, when the switching valve 35 is in its second switching state. Furthermore, a bypass 195 branches off from the junction 193 which has a differential pressure valve 197 and arranged parallel to the cooler 183. The differential pressure valve 197 opens the bypass in the direction of the volume control valve 185 under overpressure. The cooler 183 can thus be bypassed.

The volume control valve 185 is designed as 4/3-way switching valve having ports 199, 201, 203, 205 and 207. The port 199 is connected to the conduit 181 via the cooler 183 and the differential pressure valve 197, respectively, as is the port 201 which is connected to the conduit 181 via a conduit 209 and a junction 211. The ports 199 and 201 thus form a common port of the flow control valve 185 because they are both connected to the conduit 181 downstream of the cooler 183. Two ports 199, 201 are shown only for sake of clarity; in actuality, only a single port, for example 199 or 201, is provided for the conduit 181 on the volume control valve 185, wherein according to an alternative embodiment, the volume control valve 185 may in fact be formed as 5/3-way switching valve with the two separate ports 199, 201. To facilitate understanding, the following discussions refer to the illustrated embodiment, bearing in mind that the ports 199 and 201 are actually only a single port that is switched accordingly. The port 203 is connected to a conduit 213 which leads via a pressure filter 215 to the tank 3. The pressure filter 215 can be bypassed by a bypass 217 with a differential pressure valve 219 that opens in the direction of the tank 3.

The port 205 of the volume control valve 185 is connected to a cooling system 221 particularly for the first clutch K1. The port 207 is connected to a second cooling system 223 particularly for the second clutch K2.

The ports of the switching valve are constructed so as to form a plurality of switching position ranges within which the respective unobstructed flow cross section does not change. A constant flow cross-section therefore exists in each of the switching position ranges, so that no change occurs at the ports 199, 201, 203, 205 and 207 when the switching valve is actuated in the respective switching position range. Preferably, the transitions between adjacent switching position ranges are narrow, in particular compared to the width of the switching position ranges, so that the largest possible switching position ranges are formed in relation to the total switching range of the switching valve. This makes it possible to safely attain a desired switching position of the switching valve and the volume control valve 185, respectively, even when the electrical control is subject to tolerances and/or when the volume control valve 185 has tolerances, without requiring an initial calibration of the volume control valve 185 at start-up. The flow cross-section is preferably nearly constant over the entire switching control range.

In a first switching position range of the volume control valve 185, as indicated in FIG. 1, the port 201 is connected to the port 203 while the ports 199, 205 and 207 are connected blind. The total hydraulic medium flow flowing in the hydraulic conduit 181 and through the cooler 183, respectively, is hence conveyed via the port 201, 203 into conduit 213 and thus into the tank 3 via the pressure filter 215.

in a second shift position range, the ports 199 and 205 are connected to each other, while the ports 201, 203 and 207 are connected blind. In this state or in every switching position of the volume control valve 185 within the second switching position range, the entire hydraulic medium flow arriving at the volume control valve 185 is supplied to the first cooling system 221.

In a third switching position range of the volume control valve 185, the ports 199 and 207 connected together. The ports 201, 203 and 205 are connected blind. In this state or in every switching position of the volume control valve 185 within the third switching position range, the entire hydraulic medium flow flowing in the conduit 181 is then supplied to the second cooling system 223.

As already stated, the volume control valve 185 is designed as a switching valve, so that no intermediate states can be set which would enable control of the volume flow to the cooling systems 221, 223 or to the pressure filter 215. However, the volume control valve 185 may be operated in pulsed mode by fast switching back and forth, i.e. a pulsed operation of the volume control valve 185, wherein at least one switching position within one of the three switching position ranges can be assumed for a short time. The time-averaged volume flow is then also controlled or regulated in this mode of operation, which is supplied to the cooling systems 221, 223 or the pressure filter 215 and hence to the tank 3.

FIG. 1 shows that additionally a hydraulic medium flow of the conduit 57 may replace the hydraulic medium flow in the conduit 181 and be supplied to the hydraulic sub-circuit 59. Alternatively, only the conduit 57 may convey hydraulic medium. It should also be mentioned that the proportional valves 101, 101′, 141, 159, 159′, 179, 179′ are each electrically proportionally adjustable particularly against a spring force.

As stated above, the conduit 57 opens into the hydraulic sub-circuit 59, more precisely into the conduit 181 downstream of the pump 9. According to an alternative, unillustrated embodiment, the conduit 57 opens into the conduit 181 preferably downstream of the cooler 183. The total flow rate through the cooler 183 is reduced by supplying the hydraulic medium from the high pressure circuit into the hydraulic sub-circuit 59 according to the alternative embodiment. The pressure drop across the cooler 183 is reduced due to the reduced volume flow, thereby also reducing the necessary drive power for the pumps 7 and/or 9. The drive energy required for driving the electric motor 5 is likewise reduced by reducing the backpressures. According to another embodiment, the pump 9 can be connected directly to the electric motor 5 with a sufficiently large reduction of the backpressures or of the pressure level—irrespective of how this reduction is achieved—, i.e. the clutch 11 can be eliminated.

According to an additional unillustrated embodiment regarding the arrangement of the pressure filter 215, the pressure filter 215 is arranged in the conduit 213 not between the volume control valve 185 and the tank 3, but preferably in the conduit 181, in particular between the cooler 183 and the volume control valve 185. Preferably, the conduit 57 opens into conduit 181 downstream of the pressure filter 215. With this alternative arrangement of the pressure filter 215, which is now in the main flow of the hydraulic medium, the fraction of time during which the hydraulic medium is filtered by the filter pressure 215 is increased. The bypass valve 219 is preferably designed for a minimum backpressure over the volume flow.

According to another embodiment and alternatively to the illustrated and described embodiment of the volume control valve 185, the switching position ranges are interchanged such that in the first switching position range the ports 199 and/or 201 are connected to the port 205 or 207 while the other ports of the volume control valve 185 are connected blind, in the second switching position range the connections 201 and/or 199 are connected to the port 3 while the other ports are connected blind, and in the third switching position range the ports 199 and/or 201 are connected to the port 207 or 205 while the remaining ports are connected blind. Interchanging the switching positions prevents, when using pulsed control for the volume control valve 185 for setting a desired hydraulic medium flow for one of the cooling systems 221 and 223, respectively, the hydraulic medium from flowing also to the other cooling system 223 or 221. Instead, the volume flow that is not conveyed to the respective cooling system 221 or 223 under pulsed operation is directed into the tank 3. In the actual design of the volume control valve 185 as a 4/3-way proportional valve, the ports 199 and 201 are always to be understood as a common or sole port for the conduit 181 to the volume control valve 185, so that in fact only one of the two ports 199, 201 is provided on the volume control valve 185.

LIST OF REFERENCE SYMBOLS

-   1 hydraulic circuit -   3 tank -   5 electric motor -   7 first pump -   9 second pump -   11 Separating element -   13 Conduit -   15 Conduit -   17 Tee, junction -   19 Conduit -   21 Suction filter -   23 Conduit -   25 junction -   27 Pressure relief valve -   29 Conduit -   31 Pressure Filter -   33 Port -   35 Switching valve -   37 bypass -   39 Differential pressure valve -   41 Port -   43 Port -   45 Port -   47 Port -   49 Conduit -   51 Check valve -   53 pressure accumulator -   55 pressure sensing device -   57 Conduit -   59 hydraulic sub-circuit -   61 Conduit -   63 Conduit -   65 valve face -   67 Valve face -   69 conduit -   71 junction -   73 conduit -   75 junction -   77 junction -   79 junction -   81 conduit -   83 conduit -   85 conduit -   87 Sub-transmission circuit -   89 Port -   91 Switching valve -   91′ Switching valve -   93 Port -   95 Port -   97 conduit -   99 Port -   101 Pressure control valve -   101′ Pressure control valve -   103 Port -   105 Conduit -   107 Port -   109 Conduit -   111 check valve -   113 junction -   115 Conduit -   117 junction -   119 Pressure sensing device -   121 pilot valve -   121′ Pilot valve -   123 Electrical control -   125 port -   127 Port -   129 Port -   131 conduit -   133 junction -   135 Conduit -   137 Valve face -   139 sub-transmission circuit -   141 Pressure control valve -   143 Conduit -   145 junction -   147 Conduit -   149 Conduit -   151 junction -   153 Conduit -   155 gear actuating cylinder -   155′ gear actuating cylinder -   157 gear actuating cylinder -   157′ gear actuating cylinder -   159 volume control valve -   159′ volume control valve -   161 port -   163 port -   165 port -   167 port -   169 chamber -   171 chamber -   173 piston -   175 junction -   177 Conduit -   179 volume control valve -   179′ volume control valve -   181 conduit -   183 cooler -   185 volume control valve -   187 junction -   189 Conduit -   191 Pressure relief valve -   193 junction -   195 Bypass -   197 Differential pressure valve -   199 port -   201 port -   203 port -   205 port -   207 port -   209 Conduit -   211 junction -   213 Conduit -   215 pressure filter -   217 Bypass -   219 Differential pressure valve -   221 cooling system -   223 cooling system -   K1 clutch -   K2 Clutch 

1-9. (canceled)
 10. A dual clutch transmission comprising two clutches and a hydraulic circuit, the hydraulic circuit comprising: at least one pump operative connected to a rotation-speed-controlled electric motor for conveying a hydraulic medium flow, at least one cooler for cooling the hydraulic medium flow, and a volume control valve for adjusting the hydraulic medium flow for a corresponding cooling system associated with each of the two clutches, wherein the volume control valve is constructed as a switching valve having at least two switching position ranges configured to convey the hydraulic medium flow in a first switching position range with a constant flow cross-section to a first cooling system associated with a first of the two clutches and to convey the hydraulic medium flow in a second switching position range with a constant flow cross-section to a second cooling system associated with a second of the two clutches, wherein a combined switching position range of the volume control valve is substantially formed by the at least two switching position ranges and by a narrow transition region located between adjacent switching position ranges of the at least two switching position ranges, wherein the narrow transition region is designed to be so narrow that a substantially constant flow cross-section results, and wherein a quantity of the hydraulic medium conveyed to at least one of the first and the second cooling system is adjusted by pulsed control of the volume control valve and by adjusting a rotation speed of the rotation-speed-controlled electric motor.
 11. The dual clutch transmission of claim 10, configured for installation in a motor vehicle.
 12. The dual clutch transmission of claim 10, wherein hydraulic circuit is configured for cooling the two clutches.
 13. The dual clutch transmission of claim 10, wherein the volume control valve is constructed as a 4/3-way switching valve or as a 3/2-way switching valve.
 14. The dual clutch transmission of claim 10, wherein the volume control valve is constructed as a 4/3-way switching valve and conveys the hydraulic medium flow in a third switching position range to a tank providing the hydraulic medium.
 15. The dual clutch transmission of claim 10, wherein the volume control valve is controlled electromagnetically or by an electric motor, or both.
 16. The dual clutch transmission of claim 10, further comprising an actuatable separation element interposed between the at least one pump and the rotation-speed-controlled electric motor.
 17. The dual clutch transmission of claim 14, wherein the second switching position range is located between the first switching position range and the third switching position range.
 18. The dual clutch transmission of claim 14, wherein the third switching position range is located between the first switching position range and the second switching position range. 